1. Field of the Invention
The present invention relates to electronic guns for electronic tubes.
The following description relates to the case of guns for travelling wave tubes, but the invention also applies to guns intended for other sorts of electronic tubes, such for example as klystrons.
Some applications of travelling wave tubes require power modulation of the electron beam. In this case a cathode is used whose emissive surface is in the shape of a concave spherical skull cap, which is followed by a modulating grid, also in the form of a concave spherical skull cap, and whose distance from the cathode is constant at all points. This modulation grid may be successively subjected to two voltages:
a beam disabling voltage, which is negative with respect to the cathode. There is no electron emission; PA1 a beam enabling voltage which is positive with respect to the cathode.
2. Description of the Prior Art
A disabling voltage of -100 V may for example be used with an enabling voltage of +100 V.
The problem which arises in this method of use is that the grid is considerably heated when it receives a positive enabling voltage of +100 V for example.
To overcome this problem of heating up of the grid, a zero enabling voltage is used and a disabling voltage a little higher in absolute value, equal for example to -300 V.
The problem which then arises is that the modulation grid vibrates strongly under the effect of the electric field.
In fact, the modulation grid is subjected to forces F proportional to the square of the electric field, and which may be expressed in the following way: F=k.(V.sup.2 /d.sup.2), where k is a factor of proportionality, V is the disabling or enabling voltage received by the grid and d is the distance between the cathode and the grid.
The use of a zero enabling voltage results both in a reduction of the distance d between the grid and the cathode, which is then of the order for example of a few hundredths of a millimeter, and an increase in the absolute value of the disabling voltage. The force F applied to the grid during disabling of the beam is then very high. The resulting vibrations have more especially the disadvantage of causing modulation of the power of the beam during the conduction phase. In some applications of travelling wave tubes, their modulation frequency covers a very wide band and it may happen that it is exactly equal to a mechanical resonance frequency of the grid. The amplitudes of the vibrations are then very high, which may cause short circuits between the grid and the cathode by placing them in contact. These vibrations may also cause destruction of the grid by exceeding the elastic limit of the material forming it.
The present invention provides a simple and efficient solution to the above mentioned problems.